Question: $ B = \left[\begin{array}{rrr}2 & 2 & 4 \\ -1 & -2 & 0\end{array}\right]$ $ C = \left[\begin{array}{rr}1 & 4 \\ 1 & 0 \\ 5 & -1\end{array}\right]$ What is $ B C$ ?
Explanation: Because $ B$ has dimensions $(2\times3)$ and $ C$ has dimensions $(3\times2)$ , the answer matrix will have dimensions $(2\times2)$ $ B C = \left[\begin{array}{rrr}{2} & {2} & {4} \\ {-1} & {-2} & {0}\end{array}\right] \left[\begin{array}{rr}{1} & \color{#DF0030}{4} \\ {1} & \color{#DF0030}{0} \\ {5} & \color{#DF0030}{-1}\end{array}\right] = \left[\begin{array}{rr}? & ? \\ ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ B$ , with the corresponding elements in column $j$ of the second matrix, $ C$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ B$ with the first element in ${\text{column }1}$ of $ C$ , then multiply the second element in ${\text{row }1}$ of $ B$ with the second element in ${\text{column }1}$ of $ C$ , and so on. Add the products together. $ \left[\begin{array}{rr}{2}\cdot{1}+{2}\cdot{1}+{4}\cdot{5} & ? \\ ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ B$ with the corresponding elements in ${\text{column }1}$ of $ C$ and add the products together. $ \left[\begin{array}{rr}{2}\cdot{1}+{2}\cdot{1}+{4}\cdot{5} & ? \\ {-1}\cdot{1}+{-2}\cdot{1}+{0}\cdot{5} & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ B$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ C$ and add the products together. $ \left[\begin{array}{rr}{2}\cdot{1}+{2}\cdot{1}+{4}\cdot{5} & {2}\cdot\color{#DF0030}{4}+{2}\cdot\color{#DF0030}{0}+{4}\cdot\color{#DF0030}{-1} \\ {-1}\cdot{1}+{-2}\cdot{1}+{0}\cdot{5} & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rr}{2}\cdot{1}+{2}\cdot{1}+{4}\cdot{5} & {2}\cdot\color{#DF0030}{4}+{2}\cdot\color{#DF0030}{0}+{4}\cdot\color{#DF0030}{-1} \\ {-1}\cdot{1}+{-2}\cdot{1}+{0}\cdot{5} & {-1}\cdot\color{#DF0030}{4}+{-2}\cdot\color{#DF0030}{0}+{0}\cdot\color{#DF0030}{-1}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rr}24 & 4 \\ -3 & -4\end{array}\right] $